Controlling air-fuel ratio in internal combustion engines can help to reduce exhaust emissions and increase engine efficiency. Various approaches for engine operation and exhaust monitoring have been developed for vehicle internal combustion engines, including air-fuel ratio control strategies based on measured exhaust air-fuel ratios downstream from the engine. For example, exhaust gas oxygen sensors upstream from an emission control device may be used to infer engine and exhaust air-fuel ratios, and exhaust gas oxygen sensors upstream and downstream from an emission control device may be used to infer useful life of the emission control device. In response to the measured air-fuel ratios, engine operating conditions can be manipulated to maintain efficient engine operation and to control exhaust emissions.
The inventors herein have recognized issues with the above approaches. Namely, during engine operating conditions with increased scavenging, air-fuel ratios measured upstream from an emission control device may not be equivalent to in-cylinder air-fuel ratios due to pass-through of intake air at the engine cylinder during scavenging and inhomogeneous mixing in the exhaust upstream of the emission control device near the engine cylinders. Thus, air-fuel sensors positioned at the exhaust upstream from an emission control device may measure leaner than actual air-fuel ratios. Furthermore, air-fuel sensors positioned at the exhaust close to the engine cylinder may also measure leaner than actual air-fuel ratios due to sensor measurement errors caused by the presence of uncombusted fuel components in the exhaust. As a result, basing engine operation on an air-fuel ratio measured upstream of an emission control device may reduce scavenging capabilities in order to maintain operation of the engine at stoichiometric air-fuel levels. Furthermore, because scavenging cylinder exhaust gas can help to increase engine efficiency, and reduce engine knocking, especially at low engine speeds, operating an engine based on air-fuel ratios measured upstream of an emission control device may reduce overall engine performance.
One approach that at least partially addresses the above issues comprises a method for an engine, comprising during a first condition comprising decreased scavenging, adjusting fuel injection based on a more-upstream air-fuel ratio, and during a second condition comprising increased scavenging, adjusting fuel injection based on one or more more-downstream air-fuel ratios and not based on the more-upstream air-fuel ratio. Furthermore, during the first condition, adjusting fuel injection may be further based on one or more more-downstream air-fuel ratios. In this manner, an internal combustion engine may be operated with increased scavenging capabilities, higher efficiency, and with reduced emissions, as compared to engine operation based on measuring air-fuel ratios solely upstream of an emission control device.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.